Operation of molten carbonate fuel cells with high electrolyte fill level

ABSTRACT

An elevated target amount of electrolyte is used to initially fill a molten carbonate fuel cell that is operated under carbon capture conditions. The increased target electrolyte fill level can be achieved in part by adding additional electrolyte to the cathode collector prior to start of operation. The increased target electrolyte fill level can provide improved fuel cell performance and lifetime when operating a molten carbonate fuel cell at high current density with a low-CO2 content cathode input stream and/or when operating a molten carbonate fuel cell at high CO2 utilization.

FIELD

Systems and methods are provided for operating molten carbonate fuelcells for enhanced CO₂ utilization while maintaining long operationallifetime. The systems and methods include using an increased fill levelof electrolyte within the fuel cell and/or associated structures.

BACKGROUND

This application discloses and claims subject matter made as a result ofactivities within the scope of a joint research agreement betweenExxonMobil Research and Engineering Company and FuelCell Energy, Inc.that was in effect on or before the effective filing date of the presentapplication.

Molten carbonate fuel cells utilize hydrogen and/or other fuels togenerate electricity. The hydrogen may be provided by reforming methaneor other reformable fuels in a steam reformer, such as steam reformerlocated upstream of the fuel cell or integrated within the fuel cell.Fuel can also be reformed in the anode cell in a molten carbonate fuelcell, which can be operated to create conditions that are suitable forreforming fuels in the anode. Still another option can be to performsome reforming both externally and internally to the fuel cell.Reformable fuels can encompass hydrocarbon materials that can be reactedwith steam and/or oxygen at elevated temperature and/or pressure toproduce a gaseous product that comprises hydrogen.

One of the attractive features of molten carbonate fuel cells is theability to transport CO₂ from a low concentration stream (such as acathode input stream) to a higher concentration stream (such as an anodeoutput flow). During operation, CO₂ and O₂ in an MCFC cathode areconverted to carbonate ion (CO₃ ²⁻), which is then transported acrossthe molten carbonate electrolyte as a charge carrier. The carbonate ionreacts with H₂ in the fuel cell anode to form H₂O and CO₂. Thus, one ofthe net outcomes of operating the MCFC is transport of CO₂ across theelectrolyte. This transport of CO₂ across the electrolyte can allow anMCFC to generate electrical power while reducing or minimizing the costand/or challenge of sequestering carbon oxides from variousCO_(x)-containing streams. When an MCFC is paired with a combustionsource, such as a natural gas fired power plant, this can allow foradditional power generation while reducing or minimizing the overall CO₂emissions that result from power generation.

U.S. Patent Application Publication 2015/0093665 describes methods foroperating a molten carbonate fuel cell with some combustion in thecathode to provide supplemental heat for performing additional reforming(and/or other endothermic reactions) within the fuel cell anode. Thepublication notes that the voltage and/or power generated by a carbonatefuel cell can start to drop rapidly as the CO₂ concentration falls belowabout 1.0 mole %. The publication further state that as the CO₂concentration drops further, e.g., to below about 0.3 vol %, at somepoint the voltage across the fuel cell can become low enough that littleor no further transport of carbonate may occur and the fuel cell ceasesto function.

An article by Manzolini et al. (Journal of Fuel Cell Science andTechnology, Vol. 9, 2012) describes a method for modeling theperformance of a power generation system using a fuel cell for CO₂separation. Various fuel cell configurations are modeled for processinga CO₂-containing exhaust from a natural gas combined cycle turbine. Thefuel cells are used to generate additional power while alsoconcentrating CO₂ in the anode exhaust of the fuel cells. The lowest CO₂concentration modeled for the cathode outlet of the fuel cells wasroughly 1.4 vol %.

U.S. Pat. No. 7,939,219 describes in-situ delayed addition of carbonateelectrolyte for a molten carbonate fuel cell. The delayed addition ofcarbonate electrolyte is achieved by including additional electrolyte inthe fuel cell that remains solid for an extended period of time, such as2000 hours or more. After the extended period of time, the additionalelectrolyte melts and replenishes the electrolyte in the fuel cell. Thisis described as providing for a longer fuel cell lifetime.

U.S. Pat. No. 8,557,468 describes molten carbonate fuel cells withelectrolytes that include multiple carbonate components and/oradditional lithium precursors. The electrolytes correspond to botheutectic and non-eutectic mixtures of alkali carbonates, optionally withother metal carbonates and/or other lithium precursors.

A journal article titled “Degradation Mechanism of Molten Carbonate FuelCell Based on Long-Term Performance: Long-Term Operation by UsingBench-Scale Cell and Post-Test Analysis of the Cell” (Journal of PowerSources, Vol. 195, Issue 20, 15 Oct. 2018) describes addition ofcarbonate electrolyte at various points after start of operation.

SUMMARY

In an aspect, a method is provided for producing electricity in a moltencarbonate fuel cell comprising a lithium-containing electrolyte. Themethod includes operating a molten carbonate fuel cell comprising ananode, a matrix, and a cathode with a cathode input stream comprising 10vol % or less of CO₂ at an average current density of 120 mA/cm² or moreand a CO₂ utilization of 60% or more. The molten carbonate fuel cellincludes a combined target electrolyte fill level of 70 vol % or more ofa combined matrix pore volume and cathode pore volume.

In another aspect, a method is provided for producing electricity in amolten carbonate fuel cell comprising a lithium-containing electrolyte.The method includes operating a molten carbonate fuel cell comprising ananode, a matrix, and a cathode with a cathode input stream comprisingCO₂ at an average current density of 120 mA/cm² or more and a CO₂utilization of 90% or more. The molten carbonate fuel cell includes acombined target electrolyte fill level of 70 vol % or more of a combinedmatrix pore volume and cathode pore volume.

In still another aspect, a molten carbonate fuel cell is provided. Thefuel cell includes a cathode collector, a cathode, a matrix, and ananode. The fuel further includes a lithium-containing electrolyte.Additionally, the fuel cell includes a combined target electrolyte filllevel of the lithium-containing electrolyte corresponding to 85 vol % ormore of a combined matrix pore volume and cathode pore volume.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows an example of a configuration for molten carbonate fuelcells and associated reforming and separation stages.

FIG. 2 shows another example of a configuration for molten carbonatefuel cells and associated reforming and separation stages.

FIG. 3 shows an example of a molten carbonate fuel cell.

FIG. 4 shows the relative operating voltage as a function of time formolten carbonate fuel cells operated under carbon capture conditionswith varying levels of target electrolyte fill in the cathode.

FIG. 5 shows the cathode lithium content for cathodes from moltencarbonate fuel cells operated with varying levels of CO₂ in the cathodeinput stream.

FIG. 6 shows the relative ohmic resistance for molten carbonate fuelcells operated under various conditions and with various targetelectrolyte fill levels in the cathode.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Overview

In various aspects, an elevated amount of electrolyte is used toinitially fill a molten carbonate fuel cell that is operated undercarbon capture conditions. The increased initial electrolyte fill levelcan be achieved in part by adding additional electrolyte to the cathodecollector prior to start of operation. The increased initial electrolytefill level can provide improved fuel cell performance and lifetime whenoperating a molten carbonate fuel cell at high current density with alow-CO₂ content cathode input stream and/or when operating a moltencarbonate fuel cell at high CO₂ utilization. This is in contrast to fuelcell operation at conventional conditions, where an elevated initialelectrolyte fill level leads to reduced operating voltage.

The initial electrolyte fill level can be characterized in several ways.One option is to characterize a combined target electrolyte fill amountfor the combined pore volume of the matrix and the cathode. A combinedtarget electrolyte fill level or amount is defined herein as the amountof the combined matrix pore volume and cathode pore volume that would beoccupied by the electrolyte if all of the initial electrolyte fillamount were in a molten state and located in the matrix or cathode. Itis understood that the target electrolyte fill level is acharacterization of the total electrolyte initially added to a moltencarbonate fuel cell. Thus, in practice, the combined amount of matrixpore volume and cathode pore volume that will actually be occupied byelectrolyte will be lower. This is due, for example, to the fact thatnot all of the electrolyte melts immediately when starting up a fuelcell, so a portion of the unmelted (solid) electrolyte will likely stillbe present in the cathode collector. As the fuel cell operates,additional electrolyte will melt, but the consumption of electrolyte bythe cathode and/or other electrolyte losses will prevent the actualcombined fill level from reaching the “target” combined fill level.

A second option can be to separately characterize the target fill levelfor the matrix pore volume and the target fill level for the cathodepore volume. It is noted that the cathode pore volume is typically1.5-2.0 times the matrix pore volume. Thus, when determining a combinedtarget fill level based on the separate target fill levels for thematrix pore volume and the cathode pore volume, the combined target filllevel corresponds to a weighted average. For example, if the cathodepore volume is 2.0 times the matrix pore volume, the combined targetfill amount can be calculated as (<matrix pore volume>+<2.0*cathode porevolume>)/3. Similarly, if the cathode pore volume is 1.5 times thematrix pore volume, the combined target fill amount can be calculated as(<matrix pore volume>+<1.5*cathode pore volume>)/2.5.

Traditionally, fuel cells have been used as a method to convert chemicalenergy into electrical energy. Operating conditions were traditionallyselected to maintain suitably high operating voltage while efficientlyproducing electric current. In order to achieve this, the cathodeoperating conditions were typically selected so that a substantialexcess of CO₂ was available. This corresponded to, for example, a CO₂concentration in the cathode input flow of 17% or more, with a CO₂utilization of 75% or less.

The amount of electrolyte used in a conventional molten carbonate fuelcell was also selected based on a desire to maintain a high operatingvoltage. Conventional electrolyte loadings for molten carbonate fuelcells typically correspond to a target fill level of greater than 90 vol% for the matrix (relative to a pore volume of the matrix) and roughly50 vol % to 60 vol % for the cathode (relative to a pore volume of thecathode). For a fuel cell with a cathode pore volume that is 2.0 timesthe matrix volume, this corresponds to a combined target fill level ofroughly 63 vol % to 73 vol % (determined as a weighted average). For afuel cell with a cathode pore volume that is 1.5 times the matrixvolume, this corresponds to a combined target fill level of roughly 66vol % to 76 vol %. Under non-carbon capture conditions, such asoperating with a CO₂ utilization of 75% or less and a CO₂ concentrationin the cathode input of 12 vol % or more, it has been found thatincreasing the target electrolyte fill level for the cathode results ina substantial decrease in operating voltage. It is noted that the amountof pore volume in the anode that is occupied by electrolyte is smallrelative to the pore volume of the cathode and/or relative to thecombined pore volume of the matrix and the cathode.

It is noted that U.S. Pat. No. 7,939,219 describes having an additional10% of the target electrolyte volume present in a fuel cell in the formof an electrolyte that remains solid until later in the operation of thecell. Based on the conventional combined target fill levels describedabove, an additional 10% of the target electrolyte volume would, atmost, correspond to an additional 7.6 vol %, resulting in a combinedtarget fill level of 84 vol % or less.

The electrolyte in a molten carbonate fuel cells typically correspondsto a mixture of lithium carbonate with one or more other alkali metalcarbonates. Conventionally, eutectic mixtures of carbonate salts areconvenient to use, as the composition of the electrolyte in solid formis the same as the composition that will melt into the fuel cell aselectrolyte stored in the cathode collector is melted into a liquid.

It has been discovered that when operating under carbon captureconditions and generating a high current density, an unexpected increasein operating voltage can be achieved by increasing the combined targetelectrolyte fill level to 70 vol % or more, or 85 vol % or more or 90vol % or more. For example, the combined target electrolyte fill levelcan be 70 vol % to 128 vol %, or 85 vol % to 128 vol %, or 90 vol % to128 vol %, or 100 vol % to 128 vol %, or 70 vol % to 115 vol %, or 85vol % to 115 vol %, or 90 vol % to 115 vol %, or 70 vol % to 100 vol %,or 85 vol % to 100 vol %, or 90 vol % to 100 vol %. This unexpectedvoltage increase when operating with an elevated combined targetelectrolyte fill level can be observed after operating the moltencarbonate fuel cell at carbon capture conditions with high currentdensity for a cumulative time of 50 hours or more, or 100 hours or more,or 200 hours or more.

In terms of the individual target fill levels, the unexpected increasein operating voltage can be achieved by using a) a target matrixelectrolyte fill level of 90 vol % to 100 vol % for the matrix porevolume and b) a target cathode electrolyte fill level of 65 vol % to 140vol % of the cathode pore volume, or 65 vol % to 120 vol %, or 65 vol %to 100 vol %, or 75 vol % to 140 vol %, or 75 vol % to 120 vol %, or 75vol % to 100 vol %, or 85 vol % to 140 vol %, or 85 vol % to 120 vol %,or 85 vol % to 100 vol %, or 95 vol % to 140 vol %, or 95 vol % to 120vol %.

During conventional operation, increasing the amount of combined targetelectrolyte fill beyond the conventional 90+ vol % of the matrix porevolume and 50 vol % to 60 vol % of the cathode pore volume results in asubstantial loss in operating voltage. However, it has been discoveredthat when operating a fuel cell under carbon capture conditions withhigh current density, using an elevated combined target electrolyte filllevel provides an unexpected operating voltage benefit over time.Additionally, using an elevated combined target electrolyte fill levelwhen operating the fuel cell under carbon capture conditions with highcurrent density can provide an unexpected increase in fuel celloperating lifetime. Carbon capture conditions, as defined herein, referto conditions where a fuel cell is operated with a CO₂ content in thecathode input stream of 10 vol % or less and/or when operating a fuelcell at a CO₂ utilization of 90 vol % or more. In some aspects, whenoperating with a cathode input stream containing 10 vol % or less ofCO₂, the CO₂ utilization can be 70 vol % or more, or 75 vol % or more,or 80 vol % or more, such as up to 95 vol % or possibly still higher.Operating a fuel cell under carbon capture conditions with high currentdensity refers to conditions where the fuel cell is operated to generatea current density of 120 mA/cm² or more while operating under carboncapture conditions, or 130 mA/cm² or more, or 140 mA/cm² or more, or 150mA/cm² or more, such as up to 300 mA/cm² or possibly still higher.

Without being bound by any particular theory, it is believed thatoperating under carbon capture conditions causes lithium in the fuelcell to be depleted at an increased rate. Some of the lithium depletionis believed to be due to evaporation or other loss outside of the cell.It is believed that such losses can be accelerated by high spacevelocities, as may often be used under carbon capture conditions. Otherlithium depletion is believed to be due to incorporation of lithium intothe fuel cell cathode and/or the matrix. Such incorporation of lithiuminto structures within the fuel cell can be thermodynamically favored atsufficiently low concentrations of CO₂. This electrolyte depletion undercarbon capture conditions can cause the electrolyte fill level in thefuel cell to be roughly 20 vol % to 30 vol % lower at end of run thanwould be expected under conventional operation. When using aconventional electrolyte loading, the increased depletion of lithiumresults in a loss of fuel cell operating voltage and lifetime.

The electrolyte loss phenomenon reduces the ionic conductivity of themelt and the active area of the cathode, which can result in unfilledpores in the matrix network. As a result, higher ohmic resistance andgas crossover have been observed after extended testing of the fuel cellat carbon capture conditions. This leads to reduced fuel cell voltageseven at modest current densities (<100 mA/cm²). Additionally, if gascrossover is occurring in appreciable amounts, this can lead to rapidfuel cell voltage decay. Gas crossover leads to the direct combustion offuel rather than the electrochemical oxidation and risks oxidation ofthe anode and the reforming catalyst stored in the anode currentcollector, impacting directly the stack temperature and the thermalprofile. This, combined with the higher voltage decay rate, leads toexcess heat generation which reduces the fuel cell operating efficiencyand further accelerates decay mechanisms such as corrosion. The effectof site deactivation is more gradual but still detrimental to the longterm health of the fuel cell and performance. Increasing the initialelectrolyte fill level can offset the additional depletion of lithiumwhen operating a fuel cell under carbon capture conditions.

Additionally or alternately, using an electrolyte with an increasedamount of lithium can also be beneficial when operating a fuel cellunder carbon capture conditions. Conventionally, eutectic mixtures ofcarbonate electrolytes have been convenient to use. Because the increasein electrolyte depletion is selective for lithium depletion, however,using an electrolyte containing a greater amount of lithium than aeutectic mixture can potentially be beneficial.

In some aspects, the carbon capture conditions can correspond toconditions where substantial transport of alternative ions occurs ascharge carriers across the electrolyte. Hydroxide ions are an example ofan alternative ion that can be transported across the electrolyte if theconcentration of CO₂ is sufficiently low in a localized region of thefuel cell. Conventional operating conditions for molten carbonate fuelcells typically correspond to conditions where the amount of alternativeion transport is reduced, minimized, or non-existent. By contrast, undercarbon capture conditions, a portion of the charge transported acrossthe electrolyte in the fuel cell can be based on transport of ions otherthan carbonate ions.

One difficulty in using MCFCs for elevated CO₂ capture is that theoperation of the fuel cell can potentially be kinetically limited if oneor more of the reactants required for fuel cell operation is present inlow quantities. For example, when using a cathode input stream with aCO₂ content of 4.0 vol % or less, achieving a CO₂ utilization of 75% ormore corresponds to a cathode outlet concentration of 1.0 vol % or less.However, a cathode outlet concentration of 1.0 vol % or less does notnecessarily mean that the CO₂ is evenly distributed throughout thecathode. Instead, the concentration will typically vary within thecathode due to a variety of factors, such as the flow patterns in theanode and the cathode. The variations in CO₂ concentration can result inportions of the cathode where CO₂ concentrations substantially below 1.0vol % are present.

Conventionally, it would be expected that depletion of CO₂ within thecathode would lead to reduced voltage and reduced current density.However, it has been discovered that current density can be maintainedas CO₂ is depleted due to ions other than CO₃ ²⁻ being transportedacross the electrolyte. For example, a portion of the ions transportedacross the electrolyte can correspond to hydroxide ions (OH⁻). Thetransport of alternative ions across the electrolyte can allow a fuelcell to maintain a target current density even though the amount of CO₂transported across the electrolyte is insufficient.

One of the advantages of transport of alternative ions across theelectrolyte is that the fuel cell can continue to operate, even though asufficient number of CO₂ molecules are not kinetically available. Thiscan allow additional CO₂ to be transferred from cathode to anode eventhough the amount of CO₂ present in the cathode would conventionally beconsidered insufficient for normal fuel cell operation. This can allowthe fuel cell to operate with a measured CO₂ utilization closer to 100%,while the calculated CO₂ utilization (based on current density) can beat least 3% greater than the measured CO₂ utilization, or at least 5%greater, or at least 10% greater, or at least 20% greater. It is notedthat alternative ion transport can allow a fuel cell to operate with acurrent density that would correspond to more than 100% calculated CO₂utilization.

The amount of alternative ion transport can be quantified based on thetransference for a fuel cell. The transference is defined as thefraction of ions transported across the molten carbonate electrolytethat correspond to carbonate ions, as opposed to hydroxide ions and/orother ions. A convenient way to determine the transference can be basedon comparing a) the measured change in CO₂ concentration at the cathodeinlet versus the cathode outlet with b) the amount of carbonate iontransport required to achieve the current density being produced by thefuel cell. It is noted that this definition for the transference assumesthat back-transport of CO₂ from the anode to the cathode is minimal. Itis believed that such back-transport is minimal for the operatingconditions described herein. For the CO₂ concentrations, the cathodeinput stream and/or cathode output stream can be sampled, with thesample diverted to a gas chromatograph for determination of the CO₂content. The average current density for the fuel cell can be measuredin any convenient manner.

Under conventional operating conditions, the transference can berelatively close to 1.0, such as 0.98 or more and/or such as havingsubstantially no alternative ion transport. A transference of 0.98 ormore means that 98% or more of the ionic charge transported across theelectrolyte corresponds to carbonate ions. It is noted that hydroxideions have a charge of −1 while carbonate ions have a charge of −2, sotwo hydroxide ions need to be transported across the electrolyte toresult in the same charge transfer as transport of one carbonate ion.

In contrast to conventional operating conditions, operating a moltencarbonate fuel cell with transference of 0.95 or less (or 0.97 or lesswhen operating with a high acidity electrolyte) can increase theeffective amount of carbonate ion transport that is achieved, eventhough a portion of the current density generated by the fuel cell isdue to transport of ions other than carbonate ions. In order to operatea fuel cell with a transference of 0.97 or less, or 0.95 or less,depletion of CO₂ has to occur within the fuel cell cathode. It has beendiscovered that such depletion of CO₂ within the cathode tends to belocalized. As a result, many regions within a fuel cell cathode canstill have sufficient CO₂ for normal operation. These regions containadditional CO₂ that would be desirable to transport across anelectrolyte, such as for carbon capture. However, the CO₂ in suchregions is typically not transported across the electrolyte whenoperating under conventional conditions. By selecting operatingconditions with a transference of 0.97 or less, or 0.95 or less, theregions with sufficient CO₂ can be used to transport additional CO₂while the depleted regions can operate based on alternative iontransport. This can increase the practical limit for the amount of CO₂captured from a cathode input stream.

Electrolyte Fill Level and Composition

The electrolyte loading within a molten carbonate fuel cell can becontrolled based on the amount of electrolyte included in the fuel cellduring initial formation of the fuel cell. For practical reasons,attempting to add electrolyte to a fuel cell after forming a fuel cellstructure is not economically attractive. Instead, fuel cells areusually constructed used, for a desired lifetime, and then disassembledwith recovery of any usable components for use in future fuel cellconstruction. As a result, the electrolyte fill level within a fuel cellcan be characterized based on the amount of electrolyte included in thefuel cell when it is constructed relative to the available pore volumein the matrix and the cathode of the fuel cell. This electrolyte filllevel at construction can be referred to as a target electrolyte filllevel. It is noted that the target electrolyte fill level refers toelectrolyte that is added to the fuel cell prior to initial operation.Thus, any electrolyte added after the beginning of fuel cell operationis be definition excluded from the target electrolyte fill level.

The electrolyte included in a molten carbonate fuel cell is a solid atambient conditions. Thus, during construction of a fuel cell, the targetfill level of the electrolyte can be included in the fuel cell as asolid. This solid electrolyte may be at least partially included instructures other than the matrix and the cathode. For example, at leasta portion of the solid electrolyte can be incorporated into the cathodecollector of the fuel cell. As the fuel cell is heated to reach thedesired operating temperature, the electrolyte can melt, which causeselectrolyte to flow toward the matrix and cathode within the fuel cell.

Commonly a cathode fill level of roughly 50 vol % to 60 vol % at thebeginning of life with a completely filled matrix (greater than 90 vol %of matrix pore volume) is targeted. As noted above, this conventionalloading corresponds to a combined target fill level of roughly 76 vol %or less, depending on the relative pore volumes of the cathode and thematrix. As the solid electrolyte melts, capillary force and the surfacetension cause the electrolyte to distribute throughout the pore networktherefore creating a high density of electrochemically active sites.With a completely filled matrix, gas crossover is minimal and theconductivity of the membrane layer is maximized Alternatively, highercathode fill levels are typically not used in order to avoid cathodeflooding. This occurs when excess electrolyte exists in the cathodelayer, increasing the gas phase mass transfer resistance through theporous electrode. Under conventional conditions, cathode flooding isknown to be detrimental to fuel cell performance.

The electrolyte fill level in a fuel cell can be characterized based ona comparison of the volume of electrolyte (based on being a liquid atthe operating temperature of the fuel cell) relative to the pore volumein the matrix and the cathode in the fuel cell. For the electrolyte, thevolume of liquid electrolyte at the operating temperature can becalculated based on the corresponding volume (or weight) of solidelectrolyte included in the fuel cell during formation of the fuel cell.With regard to the available pore volume, both the matrix and thecathode in a fuel cell correspond to porous structures. For example, thematrix can correspond to a porous structure that is suitable for holdingthe molten carbonate electrolyte. An example of a suitable matrixmaterial is a matrix composed of aluminum oxide and lithium aluminate.An example of a suitable cathode material is nickel oxide. The porevolume of these structures can be characterized using a convenientporosimetry method. In this discussion, the pore volume of a layer(matrix, cathode, anode) can be determined by mercury porosimetry, suchas by ASTM D4284.

Conventionally, the target electrolyte fill level within a fuel cell isselected in order to provide a balance between having sufficientelectrolyte in the cathode to provide good electrical conductivity whilealso having sufficient void space in the cathode so that CO₂ and O₂ gascan enter the porous cathode for conversion into carbonate ions.Conventionally, this corresponds to having a combined target electrolytefill level of 76 vol % or less, which corresponds to 50 vol % to 60 vol% of the available pore volume in the cathode, along with fillingsubstantially all of the available pore volume in the electrolyte matrix(greater than 90 vol %). These fill levels can be achieved by includingsufficient amounts of solid electrolyte in the matrix, the cathode,and/or the cathode collector prior to starting operation of the fuelcell.

In some aspects, any convenient type of electrolyte suitable foroperation of a molten carbonate fuel cell can be used. Many conventionalMCFCs use a eutectic carbonate mixture as the carbonate electrolyte,such as a eutectic mixture of 62 mol % lithium carbonate and 38 mol %potassium carbonate (62% Li₂CO₃/38% K₂CO₃) or a eutectic mixture of 52mol % lithium carbonate and 48 mol % sodium carbonate (52% Li₂CO₃/48%Na₂CO₃). Other eutectic mixtures are also available, such as a eutecticmixture of 40 mol % lithium carbonate and 60 mol % potassium carbonate(40 mol % Li₂CO₃/60 mol % K₂CO₃) or ternary eutectic Li/Na/K (44 mol %Li₂CO₃/30 mol % Na₂CO₃/26 mol % K₂CO₃) or any binary eutectic Li/Na (52mol % Li₂CO₃/48 mol % Na₂CO₃) doped with K₂CO₃ and/or Cs₂CO₃ and/orRb₂CO₃.

Still other eutectic mixtures can be based on combinations of three ormore carbonates, including eutectic mixtures containing three or morealkali metal carbonates. Yet other mixtures can be based on combinationsof three or more carbonates, so that the mixture differs from a eutecticmixture. Additionally or alternately, still other mixtures can includeone or more lithium precursors different from lithium carbonate.

In aspects where three or more carbonates are included in theelectrolyte, the electrolyte can include a mixture of three or more ofLi₂CO₃, Na₂CO₃, K₂CO₃, Rb₂CO₃, Cs₂CO₃, BaCO₃, La₂O₃, Bi₂O₃, Bi₂O₅,Ta₂O₅, and mixtures thereof. In some aspects, 70 wt % or more, or 80 wt% or more, or 90 wt % or more, such as up to substantially all of thealkali metal carbonates in the electrolyte can correspond to a mixtureof two or more of Li₂CO₃, Na₂CO₃, and K₂CO₃. Preferably, 65 wt % ormore, or 80 wt % or more, or 90 wt % or more, such as up tosubstantially all of the electrolyte can correspond to alkali metalcarbonates. In aspects where a lithium precursor material is included,the lithium precursor material can optionally but preferably be one ormore of lithium hydroxide, lithium nitrate, lithium acetate, lithiumoxalate and mixtures thereof.

While eutectic mixtures of carbonate can be convenient as an electrolytefor various reasons, in some aspects non-eutectic mixtures of carbonatescan be advantageous. In particular, because lithium is selectively lostunder carbon capture conditions, it is believed that using anon-eutectic mixture of carbonates with more lithium carbonate than theeutectic point can be beneficial. In this discussion, the differencebetween the composition for a mixture of carbonates and a eutecticcomposition can be characterized based on the difference in the weightpercentage of lithium carbonate in the mixture versus the weightpercentage of lithium carbonate in the corresponding eutectic mixture.For determining the corresponding eutectic mixture, all alkali metalcarbonates are included, but non-alkali metal carbonates that arepresent in an amount of 2 wt % or less are not considered. As anexample, if a mixture of 80 wt % lithium carbonate and 20 wt % sodiumcarbonate is used, the mixture can be characterized as having a lithiumcarbonate content that differs from the corresponding eutectic mixtureby 28 wt %. Generally, non-eutectic mixtures can include variouscombinations of any of the carbonates and/or lithium precursor materialsdescribed herein.

In some aspects, the target electrolyte fill level can be based onincluding a plurality of types of carbonate mixtures in the fuel cell.For example, non-eutectic mixtures are known to melt more slowly underfuel cell operating conditions than eutectic mixtures. Therefore, onestrategy can be to have a first portion of the electrolyte (located inthe matrix and/or cathode) that corresponds to a eutectic mixture, whilea second portion of the electrolyte (located in the cathode collector)that corresponds to a non-eutectic mixture with an increased amount oflithium relative to the eutectic mixture. Using this type of strategy,the slower melting non-eutectic mixture will have a higher lithiumcontent than the initial electrolyte, and therefore can compensate forthe selective loss of lithium during operation under carbon captureconditions. Alternatively, two non-eutectic mixtures can be used, withthe second mixture being higher in lithium carbonate content than thefirst mixture. Depending on the aspect, the amount of the firstelectrolyte mixture (i.e., the electrolyte mixture lower in lithiumcarbonate content, such as a eutectic mixture) can correspond to 20 wt %to 80 wt % of the total amount of electrolyte in the initial electrolytefill level, or 20 wt % to 50 wt %, or 55 wt % to 80 wt %.

In this discussion, a fuel cell can correspond to a single cell, with ananode and a cathode separated by an electrolyte. The anode and cathodecan receive input gas flows to facilitate the respective anode andcathode reactions for transporting charge across the electrolyte andgenerating electricity. A fuel cell stack can represent a plurality ofcells in an integrated unit. Although a fuel cell stack can includemultiple fuel cells, the fuel cells can typically be connected inparallel and can function (approximately) as if they collectivelyrepresented a single fuel cell of a larger size. When an input flow isdelivered to the anode or cathode of a fuel cell stack, the fuel stackcan include flow channels for dividing the input flow between each ofthe cells in the stack and flow channels for combining the output flowsfrom the individual cells. In this discussion, a fuel cell array can beused to refer to a plurality of fuel cells (such as a plurality of fuelcell stacks) that are arranged in series, in parallel, or in any otherconvenient manner (e.g., in a combination of series and parallel). Afuel cell array can include one or more stages of fuel cells and/or fuelcell stacks, where the anode/cathode output from a first stage may serveas the anode/cathode input for a second stage. It is noted that theanodes in a fuel cell array do not have to be connected in the same wayas the cathodes in the array. For convenience, the input to the firstanode stage of a fuel cell array may be referred to as the anode inputfor the array, and the input to the first cathode stage of the fuel cellarray may be referred to as the cathode input to the array. Similarly,the output from the final anode/cathode stage may be referred to as theanode/cathode output from the array.

It should be understood that reference to use of a fuel cell hereintypically denotes a “fuel cell stack” composed of individual fuel cells,and more generally refers to use of one or more fuel cell stacks influid communication. Individual fuel cell elements (plates) cantypically be “stacked” together in a rectangular array called a “fuelcell stack”. This fuel cell stack can typically take a feed stream anddistribute reactants among all of the individual fuel cell elements andcan then collect the products from each of these elements. When viewedas a unit, the fuel cell stack in operation can be taken as a whole eventhough composed of many (often tens or hundreds) of individual fuel cellelements. These individual fuel cell elements can typically have similarvoltages (as the reactant and product concentrations are similar), andthe total power output can result from the summation of all of theelectrical currents in all of the cell elements, when the elements areelectrically connected in series. Stacks can also be arranged in aseries arrangement to produce high voltages. A parallel arrangement canboost the current. If a sufficiently large volume fuel cell stack isavailable to process a given exhaust flow, the systems and methodsdescribed herein can be used with a single molten carbonate fuel cellstack. In other aspects of the invention, a plurality of fuel cellstacks may be desirable or needed for a variety of reasons.

For the purposes of this invention, unless otherwise specified, the term“fuel cell” should be understood to also refer to and/or is defined asincluding a reference to a fuel cell stack composed of set of one ormore individual fuel cell elements for which there is a single input andoutput, as that is the manner in which fuel cells are typically employedin practice. Similarly, the term fuel cells (plural), unless otherwisespecified, should be understood to also refer to and/or is defined asincluding a plurality of separate fuel cell stacks. In other words, allreferences within this document, unless specifically noted, can referinterchangeably to the operation of a fuel cell stack as a “fuel cell”.For example, the volume of exhaust generated by a commercial scalecombustion generator may be too large for processing by a fuel cell(i.e., a single stack) of conventional size. In order to process thefull exhaust, a plurality of fuel cells (i.e., two or more separate fuelcells or fuel cell stacks) can be arranged in parallel, so that eachfuel cell can process (roughly) an equal portion of the combustionexhaust. Although multiple fuel cells can be used, each fuel cell cantypically be operated in a generally similar manner, given its (roughly)equal portion of the combustion exhaust.

Example of Molten Carbonate Fuel Cell Structure

FIG. 3 shows a general example of a molten carbonate fuel cell. The fuelcell represented in FIG. 3 corresponds to a fuel cell that is part of afuel cell stack. In order to isolate the fuel cell from adjacent fuelcells in the stack, the fuel cell includes separator plates 310 and 311.In FIG. 3, the fuel cell includes an anode 330 and a cathode 350 thatare separated by an electrolyte matrix 340 that contains an electrolyte342. Anode collector 320 provides electrical contact between anode 330and the other anodes in the stack, while cathode collector 360 providessimilar electrical contact between cathode 350 and the other cathodes inthe fuel cell stack. Additionally, anode collector 320 allows forintroduction and exhaust of gases from anode 330, while cathodecollector 360 allows for introduction and exhaust of gases from cathode350.

For the initial electrolyte fill, solid electrolyte can be incorporated,as possible, within the matrix, the cathode, and the cathode collector.Because the electrolyte is solid during initial fill, it can bedifficult to achieve a desired loading by only adding the solidelectrolyte to the matrix and the cathode. In order to achieve a desiredloading, solid electrolyte can also be added to the cathode collector.The electrolyte added to the cathode collector can melt as the fuel cellis operated, which then allows the electrolyte to flow into the cathode.Similarly, as electrolyte in the cathode is melted, a portion of themolten electrolyte can be passed from the cathode to the matrix to filladditional portions of the matrix volume.

It is noted that practical considerations can also limit the amount ofsolid electrolyte that is added to the cathode collector. Because thesolid electrolyte melts over time, if the loading of solid electrolytein the cathode collector is too high, the ability for gas to flowthrough the cathode collector to reach the cathode may be limited. Ithas been discovered that target electrolyte loading of electrolyte of upto 140 vol % of the cathode pore volume can be used while having minimalimpact on gas transfer by using an off-eutectic composition in thecathode current collector. However, further addition of electrolyte canpotentially limit gas transfer in an undesirable manner. Relative to theavailable surface area in the fuel cell, this can correspond to a targetloading of 66 grams or less of electrolyte per 250 cm² of fuel cellarea. In some aspects, the target loading can be 40 grams to 66 grams ofelectrolyte per 250 cm² of fuel cell area, or 45 grams to 66 grams, or50 grams to 66 grams. It is noted that a portion of the targetelectrolyte loading can be included in the cathode collector. Theportion of the target electrolyte loading included in the cathodecollector can correspond to 38 grams of electrolyte or less per 250 cm²of fuel cell area. In some aspects, the portion of the targetelectrolyte loading included in the cathode collector can correspond to18 grams to 38 grams of electrolyte per 250 cm² of fuel cell area, or 24grams to 38 grams, or 28 grams to 38 grams.

During operation, CO₂ is passed into the cathode collector 360 alongwith O₂. The CO₂ and O₂ diffuse into the porous cathode 350 and travelto a cathode interface region near the boundary of cathode 350 andelectrolyte matrix 340. In the cathode interface region, a portion ofelectrolyte 342 can be present in the pores of cathode 350. The CO₂ andO₂ can be converted near/in the cathode interface region to carbonateion (CO₃ ²⁻), which can then be transported across electrolyte 342 (andtherefore across electrolyte matrix 340) to facilitate generation ofelectrical current. In aspects where alternative ion transport isoccurring, a portion of the O₂ can be converted to an alternative ion,such as a hydroxide ion or a peroxide ion, for transport in electrolyte342. After transport across the electrolyte 342, the carbonate ion (oralternative ion) can reach an anode interface region near the boundaryof electrolyte matrix 340 and anode 330. The carbonate ion can beconverted back to CO₂ and H₂O in the presence of H₂, releasing electronsthat are used to form the current generated by the fuel cell. The H₂and/or a hydrocarbon suitable for forming H₂ are introduced into anode330 via anode collector 320.

The flow direction within the anode of a molten carbonate fuel cell canhave any convenient orientation relative to the flow direction within acathode. One option can be to use a cross-flow configuration, so thatthe flow direction within the anode is roughly at a 90° angle relativeto the flow direction within the cathode. This type of flowconfiguration can have practical benefits, as using a cross-flowconfiguration can allow the manifolds and/or piping for the anodeinlets/outlets to be located on different sides of a fuel cell stackfrom the manifolds and/or piping for the cathode inlets/outlets.

Conditions for Molten Carbonate Fuel Operation

When operating a molten carbonate fuel cell to perform carbon capture,optionally with a current density of 120 mA/cm² or more, suitableconditions for the anode can include providing the anode with H₂, areformable fuel, or a combination thereof; and operating with anyconvenient fuel utilization that generates a desired current density,including fuel utilizations ranging from 20% to 80%. In some aspectsthis can correspond to a traditional fuel utilization amount, such as afuel utilization of 60% or more, or 70% or more, such as up to 85% orpossibly still higher. In other aspects, this can correspond to a fuelutilization selected to provide an anode output stream with an elevatedcontent of H₂ and/or an elevated combined content of H₂ and CO (i.e.,syngas), such as a fuel utilization of 55% or less, or 50% or less, or40% or less, such as down to 20% or possibly still lower. The H₂ contentin the anode output stream and/or the combined content of H₂ and CO inthe anode output stream can be sufficient to allow generation of adesired current density. In some aspects, the H₂ content in the anodeoutput stream can be 3.0 vol % or more, or 5.0 vol % or more, or 8.0 vol% or more, such as up to 15 vol % or possibly still higher. Additionallyor alternately, the combined amount of H₂ and CO in the anode outputstream can be 4.0 vol % or more, or 6.0 vol % or more, or 10 vol % ormore, such as up to 20 vol % or possibly still higher. Optionally, whenthe fuel cell is operated with low fuel utilization, the H₂ content inthe anode output stream can be in a higher range, such as an H₂ contentof 10 vol % to 25 vol %. In such aspects, the syngas content of theanode output stream can be correspondingly higher, such as a combined H₂and CO content of 15 vol % to 35 vol %. Depending on the aspect, theanode can be operated to increase the amount of electrical energygenerated, to increase the amount of chemical energy generated, (i.e.,H₂ generated by reforming that is available in the anode output stream),or operated using any other convenient strategy that is compatible withoperating the fuel cell to cause alternative ion transport.

In various aspects, the anode input stream for a MCFC can includehydrogen, a hydrocarbon such as methane, a hydrocarbon orhydrocarbon-like compound that may contain heteroatoms different from Cand H, or a combination thereof. The source of thehydrogen/hydrocarbon/hydrocarbon-like compounds can be referred to as afuel source. In some aspects, most of the methane (or other hydrocarbon,hydrocarbon, or hydrocarbon-like compound) fed to the anode cantypically be fresh methane. In this description, a fresh fuel such asfresh methane refers to a fuel that is not recycled from another fuelcell process. For example, methane recycled from the anode outlet streamback to the anode inlet may not be considered “fresh” methane, and caninstead be described as reclaimed methane.

The fuel source used can be shared with other components, such as aturbine that uses a portion of the fuel source to provide aCO₂-containing stream for the cathode input. The fuel source input caninclude water in a proportion to the fuel appropriate for reforming thehydrocarbon (or hydrocarbon-like) compound in the reforming section thatgenerates hydrogen. For example, if methane is the fuel input forreforming to generate H₂, the molar ratio of water to fuel can be fromabout one to one to about ten to one, such as at least about two to one.A ratio of four to one or greater is typical for external reforming, butlower values can be typical for internal reforming. To the degree thatH₂ is a portion of the fuel source, in some optional aspects noadditional water may be needed in the fuel, as the oxidation of H₂ atthe anode can tend to produce H₂O that can be used for reforming thefuel. The fuel source can also optionally contain components incidentalto the fuel source (e.g., a natural gas feed can contain some content ofCO₂ as an additional component). For example, a natural gas feed cancontain CO₂, N₂, and/or other inert (noble) gases as additionalcomponents. Optionally, in some aspects the fuel source may also containCO, such as CO from a recycled portion of the anode exhaust. Anadditional or alternate potential source for CO in the fuel into a fuelcell assembly can be CO generated by steam reforming of a hydrocarbonfuel performed on the fuel prior to entering the fuel cell assembly.

More generally, a variety of types of fuel streams may be suitable foruse as an anode input stream for the anode of a molten carbonate fuelcell. Some fuel streams can correspond to streams containinghydrocarbons and/or hydrocarbon-like compounds that may also includeheteroatoms different from C and H. In this discussion, unless otherwisespecified, a reference to a fuel stream containing hydrocarbons for anMCFC anode is defined to include fuel streams containing suchhydrocarbon-like compounds. Examples of hydrocarbon (includinghydrocarbon-like) fuel streams include natural gas, streams containingC₁-C₄ carbon compounds (such as methane or ethane), and streamscontaining heavier C₅₊ hydrocarbons (including hydrocarbon-likecompounds), as well as combinations thereof. Still other additional oralternate examples of potential fuel streams for use in an anode inputcan include biogas-type streams, such as methane produced from natural(biological) decomposition of organic material.

In some aspects, a molten carbonate fuel cell can be used to process aninput fuel stream, such as a natural gas and/or hydrocarbon stream, witha low energy content due to the presence of diluent compounds. Forexample, some sources of methane and/or natural gas are sources that caninclude substantial amounts of either CO₂ or other inert molecules, suchas nitrogen, argon, or helium. Due to the presence of elevated amountsof CO₂ and/or inert components, the energy content of a fuel streambased on the source can be reduced. Using a low energy content fuel fora combustion reaction (such as for powering a combustion-poweredturbine) can pose difficulties. However, a molten carbonate fuel cellcan generate power based on a low energy content fuel source with areduced or minimal impact on the efficiency of the fuel cell. Thepresence of additional gas volume can require additional heat forraising the temperature of the fuel to the temperature for reformingand/or the anode reaction. Additionally, due to the equilibrium natureof the water gas shift reaction within a fuel cell anode, the presenceof additional CO₂ can have an impact on the relative amounts of H₂ andCO present in the anode output. However, the inert compounds otherwisecan have only a minimal direct impact on the reforming and anodereactions. The amount of CO₂ and/or inert compounds in a fuel stream fora molten carbonate fuel cell, when present, can be at least about 1 vol%, such as at least about 2 vol %, or at least about 5 vol %, or atleast about 10 vol %, or at least about 15 vol %, or at least about 20vol %, or at least about 25 vol %, or at least about 30 vol %, or atleast about 35 vol %, or at least about 40 vol %, or at least about 45vol %, or at least about 50 vol %, or at least about 75 vol %.Additionally or alternately, the amount of CO₂ and/or inert compounds ina fuel stream for a molten carbonate fuel cell can be about 90 vol % orless, such as about 75 vol % or less, or about 60 vol % or less, orabout 50 vol % or less, or about 40 vol % or less, or about 35 vol % orless.

Yet other examples of potential sources for an anode input stream cancorrespond to refinery and/or other industrial process output streams.For example, coking is a common process in many refineries forconverting heavier compounds to lower boiling ranges. Coking typicallyproduces an off-gas containing a variety of compounds that are gases atroom temperature, including CO and various C₁-C₄ hydrocarbons. Thisoff-gas can be used as at least a portion of an anode input stream.Other refinery off-gas streams can additionally or alternately besuitable for inclusion in an anode input stream, such as light ends(C₁-C₄) generated during cracking or other refinery processes. Stillother suitable refinery streams can additionally or alternately includerefinery streams containing CO or CO₂ that also contain H₂ and/orreformable fuel compounds.

Still other potential sources for an anode input can additionally oralternately include streams with increased water content. For example,an ethanol output stream from an ethanol plant (or another type offermentation process) can include a substantial portion of H₂O prior tofinal distillation. Such H₂O can typically cause only minimal impact onthe operation of a fuel cell. Thus, a fermentation mixture of alcohol(or other fermentation product) and water can be used as at least aportion of an anode input stream.

Biogas, or digester gas, is another additional or alternate potentialsource for an anode input. Biogas may primarily comprise methane and CO₂and is typically produced by the breakdown or digestion of organicmatter. Anaerobic bacteria may be used to digest the organic matter andproduce the biogas. Impurities, such as sulfur-containing compounds, maybe removed from the biogas prior to use as an anode input.

The output stream from an MCFC anode can include H₂O, CO₂, CO, and H₂.Optionally, the anode output stream could also have unreacted fuel (suchas H₂ or CH₄) or inert compounds in the feed as additional outputcomponents. Instead of using this output stream as a fuel source toprovide heat for a reforming reaction or as a combustion fuel forheating the cell, one or more separations can be performed on the anodeoutput stream to separate the CO₂ from the components with potentialvalue as inputs to another process, such as H₂ or CO. The H₂ and/or COcan be used as a syngas for chemical synthesis, as a source of hydrogenfor chemical reaction, and/or as a fuel with reduced greenhouse gasemissions.

The anode exhaust can be subjected to a variety of gas processingoptions, including water-gas shift and separation of the components fromeach other. Two general anode processing schemes are shown in FIGS. 1and 2.

FIG. 1 schematically shows an example of a reaction system for operatinga fuel cell array of molten carbonate fuel cells in conjunction with achemical synthesis process. In FIG. 1, a fuel stream 105 is provided toa reforming stage (or stages) 110 associated with the anode 127 of afuel cell 120, such as a fuel cell that is part of a fuel cell stack ina fuel cell array. The reforming stage 110 associated with fuel cell 120can be internal to a fuel cell assembly. In some optional aspects, anexternal reforming stage (not shown) can also be used to reform aportion of the reformable fuel in an input stream prior to passing theinput stream into a fuel cell assembly. Fuel stream 105 can preferablyinclude a reformable fuel, such as methane, other hydrocarbons, and/orother hydrocarbon-like compounds such as organic compounds containingcarbon-hydrogen bonds. Fuel stream 105 can also optionally contain H₂and/or CO, such as H₂ and/or CO provided by optional anode recyclestream 185. It is noted that anode recycle stream 185 is optional, andthat in many aspects no recycle stream is provided from the anodeexhaust 125 back to anode 127, either directly or indirectly viacombination with fuel stream 105 or reformed fuel stream 115. Afterreforming, the reformed fuel stream 115 can be passed into anode 127 offuel cell 120. A CO₂ and O₂-containing stream 119 can also be passedinto cathode 129. A flow of carbonate ions 122, CO₃ ²⁻, from the cathodeportion 129 of the fuel cell can provide the remaining reactant neededfor the anode fuel cell reactions. Based on the reactions in the anode127, the resulting anode exhaust 125 can include H₂O, CO₂, one or morecomponents corresponding to incompletely reacted fuel (H₂, CO, CH₄, orother components corresponding to a reformable fuel), and optionally oneor more additional nonreactive components, such as N₂ and/or othercontaminants that are part of fuel stream 105. The anode exhaust 125 canthen be passed into one or more separation stages. For example, a CO₂removal stage 140 can correspond to a cryogenic CO₂ removal system, anamine wash stage for removal of acid gases such as CO₂, or anothersuitable type of CO₂ separation stage for separating a CO₂ output stream143 from the anode exhaust. Optionally, the anode exhaust can first bepassed through a water gas shift reactor 130 to convert any CO presentin the anode exhaust (along with some H₂O) into CO₂ and H₂ in anoptionally water gas shifted anode exhaust 135. Depending on the natureof the CO₂ removal stage, a water condensation or removal stage 150 maybe desirable to remove a water output stream 153 from the anode exhaust.Though shown in FIG. 1 after the CO₂ separation stage 140, it mayoptionally be located before the CO₂ separation stage 140 instead.Additionally, an optional membrane separation stage 160 for separationof H₂ can be used to generate a high purity permeate stream 163 of H₂.The resulting retentate stream 166 can then be used as an input to achemical synthesis process. Stream 166 could additionally or alternatelybe shifted in a second water-gas shift reactor 131 to adjust the H₂, CO,and CO₂ content to a different ratio, producing an output stream 168 forfurther use in a chemical synthesis process. In FIG. 1, anode recyclestream 185 is shown as being withdrawn from the retentate stream 166,but the anode recycle stream 185 could additionally or alternately bewithdrawn from other convenient locations in or between the variousseparation stages. The separation stages and shift reactor(s) couldadditionally or alternately be configured in different orders, and/or ina parallel configuration. Finally, a stream with a reduced content ofCO₂ 139 can be generated as an output from cathode 129. For the sake ofsimplicity, various stages of compression and heat addition/removal thatmight be useful in the process, as well as steam addition or removal,are not shown.

As noted above, the various types of separations performed on the anodeexhaust can be performed in any convenient order. FIG. 2 shows anexample of an alternative order for performing separations on an anodeexhaust. In FIG. 2, anode exhaust 125 can be initially passed intoseparation stage 260 for removing a portion 263 of the hydrogen contentfrom the anode exhaust 125. This can allow, for example, reduction ofthe H₂ content of the anode exhaust to provide a retentate 266 with aratio of H₂ to CO closer to 2:1. The ratio of H₂ to CO can then befurther adjusted to achieve a desired value in a water gas shift stage230. The water gas shifted output 235 can then pass through CO₂separation stage 240 and water removal stage 250 to produce an outputstream 275 suitable for use as an input to a desired chemical synthesisprocess. Optionally, output stream 275 could be exposed to an additionalwater gas shift stage (not shown). A portion of output stream 275 canoptionally be recycled (not shown) to the anode input. Of course, stillother combinations and sequencing of separation stages can be used togenerate a stream based on the anode output that has a desiredcomposition. For the sake of simplicity, various stages of compressionand heat addition/removal that might be useful in the process, as wellas steam addition or removal, are not shown.

Cathode Inputs and Outputs

When operating under carbon capture conditions, suitable conditions forthe cathode can include providing the cathode with cathode input flowthat includes CO₂ and O₂. In aspects where the carbon capture conditionscorrespond to conditions where alternative ion transport occurs, thecathode input flow can further include a sufficient amount of water.

The CO₂ concentration in the cathode input flow can be 10 vol % or less,or 8.0 vol % or less, or 6.0 vol % or less, or 4.0 vol % or less, suchas down to 1.5 vol % or possibly still lower. Additionally oralternately, the cathode can be operated at a CO₂ utilization of 60% ormore, or 70% or more, or 80% or more, such as up to 95% or possiblystill higher. It is noted that if the CO₂ utilization is less than 80%,then the CO₂ concentration in the cathode input flow can be 10 vol % orless. In some aspects, the O₂ concentration in the cathode input streamcan correspond to an oxygen content of 4.0 vol % to 15 vol %, or 6.0 vol% to 10 vol %.

In aspects where the carbon capture conditions correspond to conditionswhere alternative ion transport occurs, it has been observed that asufficient amount of water should also be present for alternative iontransport to occur. This can correspond to having 1.0 vol % or more ofwater present in the cathode input flow, or 2.0 vol % or more. It isnoted that because air is commonly used as an O₂ source, and since H₂Ois one of the products generated during combustion (a common source ofCO₂), a sufficient amount of water is typically available within thecathode.

Conventionally, a molten carbonate fuel cell can be operated based ondrawing a desired load while consuming some portion of the fuel in thefuel stream delivered to the anode. The voltage of the fuel cell canthen be determined by the load, fuel input to the anode, air and CO₂provided to the cathode, and the internal resistances of the fuel cell.The CO₂ to the cathode can be conventionally provided in part by usingthe anode exhaust as at least a part of the cathode input stream. Bycontrast, the present invention can use separate/different sources forthe anode input and cathode input. By removing any direct link betweenthe composition of the anode input flow and the cathode input flow,additional options become available for operating the fuel cell, such asto generate excess synthesis gas, to improve capture of carbon dioxide,and/or to improve the total efficiency (electrical plus chemical power)of the fuel cell, among others.

One example of a suitable CO₂-containing stream for use as a cathodeinput flow can be an output or exhaust flow from a combustion source.Examples of combustion sources include, but are not limited to, sourcesbased on combustion of natural gas, combustion of coal, and/orcombustion of other hydrocarbon-type fuels (including biologicallyderived fuels). Additional or alternate sources can include other typesof boilers, fired heaters, furnaces, and/or other types of devices thatburn carbon-containing fuels in order to heat another substance (such aswater or air).

Other potential sources for a cathode input stream can additionally oralternately include sources of bio-produced CO₂. This can include, forexample, CO₂ generated during processing of bio-derived compounds, suchas CO₂ generated during ethanol production. An additional or alternateexample can include CO₂ generated by combustion of a bio-produced fuel,such as combustion of lignocellulose. Still other additional oralternate potential CO₂ sources can correspond to output or exhauststreams from various industrial processes, such as CO₂-containingstreams generated by plants for manufacture of steel, cement, and/orpaper.

Yet another additional or alternate potential source of CO₂ can beCO₂-containing streams from a fuel cell. The CO₂-containing stream froma fuel cell can correspond to a cathode output stream from a differentfuel cell, an anode output stream from a different fuel cell, a recyclestream from the cathode output to the cathode input of a fuel cell,and/or a recycle stream from an anode output to a cathode input of afuel cell. For example, an MCFC operated in standalone mode underconventional conditions can generate a cathode exhaust with a CO₂concentration of at least about 5 vol %. Such a CO₂-containing cathodeexhaust could be used as a cathode input for an MCFC operated accordingto an aspect of the invention. More generally, other types of fuel cellsthat generate a CO₂ output from the cathode exhaust can additionally oralternately be used, as well as other types of CO₂-containing streamsnot generated by a “combustion” reaction and/or by a combustion-poweredgenerator. Optionally but preferably, a CO₂-containing stream fromanother fuel cell can be from another molten carbonate fuel cell. Forexample, for molten carbonate fuel cells connected in series withrespect to the cathodes, the output from the cathode for a first moltencarbonate fuel cell can be used as the input to the cathode for a secondmolten carbonate fuel cell.

In addition to CO₂, a cathode input stream can include O₂ to provide thecomponents necessary for the cathode reaction. Some cathode inputstreams can be based on having air as a component. For example, acombustion exhaust stream can be formed by combusting a hydrocarbon fuelin the presence of air. Such a combustion exhaust stream, or anothertype of cathode input stream having an oxygen content based on inclusionof air, can have an oxygen content of about 20 vol % or less, such asabout 15 vol % or less, or about 10 vol % or less. Additionally oralternately, the oxygen content of the cathode input stream can be atleast about 4 vol %, such as at least about 6 vol %, or at least about 8vol %. More generally, a cathode input stream can have a suitablecontent of oxygen for performing the cathode reaction. In some aspects,this can correspond to an oxygen content of about 5 vol % to about 15vol %, such as from about 7 vol % to about 9 vol %. For many types ofcathode input streams, the combined amount of CO₂ and O₂ can correspondto less than about 21 vol % of the input stream, such as less than about15 vol % of the stream or less than about 10 vol % of the stream. An airstream containing oxygen can be combined with a CO₂ source that has lowoxygen content. For example, the exhaust stream generated by burningcoal may include a low oxygen content that can be mixed with air to forma cathode inlet stream.

In addition to CO₂ and O₂, a cathode input stream can also be composedof inert/nonreactive species such as N₂, H₂O, and other typical oxidant(air) components. For example, for a cathode input derived from anexhaust from a combustion reaction, if air is used as part of theoxidant source for the combustion reaction, the exhaust gas can includetypical components of air such as N₂, H₂O, and other compounds in minoramounts that are present in air. Depending on the nature of the fuelsource for the combustion reaction, additional species present aftercombustion based on the fuel source may include one or more of H₂O,oxides of nitrogen (NO_(x)) and/or sulfur (SO_(x)), and other compoundseither present in the fuel and/or that are partial or completecombustion products of compounds present in the fuel, such as CO. Thesespecies may be present in amounts that do not poison the cathodecatalyst surfaces though they may reduce the overall cathode activity.Such reductions in performance may be acceptable, or species thatinteract with the cathode catalyst may be reduced to acceptable levelsby known pollutant removal technologies.

The amount of O₂ present in a cathode input stream (such as an inputcathode stream based on a combustion exhaust) can advantageously besufficient to provide the oxygen needed for the cathode reaction in thefuel cell. Thus, the volume percentage of O₂ can advantageously be atleast 0.5 times the amount of CO₂ in the exhaust. Optionally, asnecessary, additional air can be added to the cathode input to providesufficient oxidant for the cathode reaction. When some form of air isused as the oxidant, the amount of N₂ in the cathode exhaust can be atleast about 78 vol %, e.g., at least about 88 vol %, and/or about 95 vol% or less. In some aspects, the cathode input stream can additionally oralternately contain compounds that are generally viewed as contaminants,such as H₂S or NH₃. In other aspects, the cathode input stream can becleaned to reduce or minimize the content of such contaminants.

A suitable temperature for operation of an MCFC can be between about450° C. and about 750° C., such as at least about 500° C., e.g., with aninlet temperature of about 550° C. and an outlet temperature of about625° C. Prior to entering the cathode, heat can be added to or removedfrom the cathode input stream, if desired, e.g., to provide heat forother processes, such as reforming the fuel input for the anode. Forexample, if the source for the cathode input stream is a combustionexhaust stream, the combustion exhaust stream may have a temperaturegreater than a desired temperature for the cathode inlet. In such anaspect, heat can be removed from the combustion exhaust prior to use asthe cathode input stream. Alternatively, the combustion exhaust could beat very low temperature, for example after a wet gas scrubber on acoal-fired boiler, in which case the combustion exhaust can be belowabout 100° C. Alternatively, the combustion exhaust could be from theexhaust of a gas turbine operated in combined cycle mode, in which thegas can be cooled by raising steam to run a steam turbine for additionalpower generation. In this case, the gas can be below about 50° C. Heatcan be added to a combustion exhaust that is cooler than desired.

Additional Molten Carbonate Fuel Cell Operating Strategies

In some aspects, when operating a MCFC to cause alternative iontransport, the anode of the fuel cell can be operated at a traditionalfuel utilization value of roughly 60% to 80%. When attempting togenerate electrical power, operating the anode of the fuel cell at arelatively high fuel utilization can be beneficial for improvingelectrical efficiency (i.e., electrical energy generated per unit ofchemical energy consumed by the fuel cell).

In some aspects, it may be beneficial to reduce the electricalefficiency of the fuel cell in order to provide other benefits, such asan increase in the amount of H₂ provided in the anode output flow. Thiscan be beneficial, for example, if it is desirable to consume excessheat generated in the fuel cell (or fuel cell stack) by performingadditional reforming and/or performing another endothermic reaction. Forexample, a molten carbonate fuel cell can be operated to provideincreased production of syngas and/or hydrogen. The heat required forperforming the endothermic reforming reaction can be provided by theexothermic electrochemical reaction in the anode for electricitygeneration. Rather than attempting to transport the heat generated bythe exothermic fuel cell reaction(s) away from the fuel cell, thisexcess heat can be used in situ as a heat source for reforming and/oranother endothermic reaction. This can result in more efficient use ofthe heat energy and/or a reduced need for additional external orinternal heat exchange. This efficient production and use of heatenergy, essentially in-situ, can reduce system complexity and componentswhile maintaining advantageous operating conditions. In some aspects,the amount of reforming or other endothermic reaction can be selected tohave an endothermic heat requirement comparable to, or even greaterthan, the amount of excess heat generated by the exothermic reaction(s)rather than significantly less than the heat requirement typicallydescribed in the prior art.

Additionally or alternately, the fuel cell can be operated so that thetemperature differential between the anode inlet and the anode outletcan be negative rather than positive. Thus, instead of having atemperature increase between the anode inlet and the anode outlet, asufficient amount of reforming and/or other endothermic reaction can beperformed to cause the output stream from the anode outlet to be coolerthan the anode inlet temperature. Further additionally or alternately,additional fuel can be supplied to a heater for the fuel cell and/or aninternal reforming stage (or other internal endothermic reaction stage)so that the temperature differential between the anode input and theanode output can be smaller than the expected difference based on therelative demand of the endothermic reaction(s) and the combinedexothermic heat generation of the cathode combustion reaction and theanode reaction for generating electrical power. In aspects wherereforming is used as the endothermic reaction, operating a fuel cell toreform excess fuel can allow for production of increased synthesis gasand/or increased hydrogen relative to conventional fuel cell operationwhile minimizing the system complexity for heat exchange and reforming.The additional synthesis gas and/or additional hydrogen can then be usedin a variety of applications, including chemical synthesis processesand/or collection/repurposing of hydrogen for use as a “clean” fuel.

The amount of heat generated per mole of hydrogen oxidized by theexothermic reaction at the anode can be substantially larger than theamount of heat consumed per mole of hydrogen generated by the reformingreaction. The net reaction for hydrogen in a molten carbonate fuel cell(H₂+½ O₂→H₂O) can have an enthalpy of reaction of about −285 kJ/mol ofhydrogen molecules. At least a portion of this energy can be convertedto electrical energy within the fuel cell. However, the difference(approximately) between the enthalpy of reaction and the electricalenergy produced by the fuel cell can become heat within the fuel cell.This quantity of energy can alternatively be expressed as the currentdensity (current per unit area) for the cell multiplied by thedifference between the theoretical maximum voltage of the fuel cell andthe actual voltage, or <current density>*(Vmax−Vact). This quantity ofenergy is defined as the “waste heat” for a fuel cell. As an example ofreforming, the enthalpy of reforming for methane (CH₄+2H₂O→4 H₂+CO₂) canbe about 250 kJ/mol of methane, or about 62 kJ/mol of hydrogenmolecules. From a heat balance standpoint, each hydrogen moleculeelectrochemically oxidized can generate sufficient heat to generate morethan one hydrogen molecule by reforming. In a conventionalconfiguration, this excess heat can result in a substantial temperaturedifference from anode inlet to anode outlet. Instead of allowing thisexcess heat to be used for increasing the temperature in the fuel cell,the excess heat can be consumed by performing a matching amount of thereforming reaction. The excess heat generated in the anode can besupplemented with the excess heat generated by the combustion reactionin the fuel cell. More generally, the excess heat can be consumed byperforming an endothermic reaction in the fuel cell anode and/or in anendothermic reaction stage heat integrated with the fuel cell.

Depending on the aspect, the amount of reforming and/or otherendothermic reaction can be selected relative to the amount of hydrogenreacted in the anode in order to achieve a desired thermal ratio for thefuel cell. As used herein, the “thermal ratio” is defined as the heatproduced by exothermic reactions in a fuel cell assembly (includingexothermic reactions in both the anode and cathode) divided by theendothermic heat demand of reforming reactions occurring within the fuelcell assembly. Expressed mathematically, the thermal ratio(TH)=Q_(EX)/Q_(EN), where Q_(EX) is the sum of heat produced byexothermic reactions and Q_(EN) is the sum of heat consumed by theendothermic reactions occurring within the fuel cell. Note that the heatproduced by the exothermic reactions can correspond to any heat due toreforming reactions, water gas shift reactions, combustion reactions(i.e., oxidation of fuel compounds) in the cathode, and/or theelectrochemical reactions in the cell. The heat generated by theelectrochemical reactions can be calculated based on the idealelectrochemical potential of the fuel cell reaction across theelectrolyte minus the actual output voltage of the fuel cell. Forexample, the ideal electrochemical potential of the reaction in a MCFCis believed to be about 1.04 V based on the net reaction that occurs inthe cell. During operation of the MCFC, the cell can typically have anoutput voltage less than 1.04 V due to various losses. For example, acommon output/operating voltage can be about 0.7 V. The heat generatedcan be equal to the electrochemical potential of the cell (i.e. ˜1.04V)minus the operating voltage. For example, the heat produced by theelectrochemical reactions in the cell can be ˜0.34 V when the outputvoltage of ˜0.7V is attained in the fuel cell. Thus, in this scenario,the electrochemical reactions would produce ˜0.7 V of electricity and˜0.34 V of heat energy. In such an example, the ˜0.7 V of electricalenergy is not included as part of Q_(EX). In other words, heat energy isnot electrical energy.

In various aspects, a thermal ratio can be determined for any convenientfuel cell structure, such as a fuel cell stack, an individual fuel cellwithin a fuel cell stack, a fuel cell stack with an integrated reformingstage, a fuel cell stack with an integrated endothermic reaction stage,or a combination thereof. The thermal ratio may also be calculated fordifferent units within a fuel cell stack, such as an assembly of fuelcells or fuel cell stacks. For example, the thermal ratio may becalculated for a fuel cell (or a plurality of fuel cells) within a fuelcell stack along with integrated reforming stages and/or integratedendothermic reaction stage elements in sufficiently close proximity tothe fuel cell(s) to be integrated from a heat integration standpoint.

From a heat integration standpoint, a characteristic width in a fuelcell stack can be the height of an individual fuel cell stack element.It is noted that the separate reforming stage and/or a separateendothermic reaction stage could have a different height in the stackthan a fuel cell. In such a scenario, the height of a fuel cell elementcan be used as the characteristic height. In this discussion, anintegrated endothermic reaction stage can be defined as a stage heatintegrated with one or more fuel cells, so that the integratedendothermic reaction stage can use the heat from the fuel cells as aheat source for reforming. Such an integrated endothermic reaction stagecan be defined as being positioned less than 10 times the height of astack element from fuel cells providing heat to the integrated stage.For example, an integrated endothermic reaction stage (such as areforming stage) can be positioned less than 10 times the height of astack element from any fuel cells that are heat integrated, or less than8 times the height of a stack element, or less than 5 times the heightof a stack element, or less than 3 times the height of a stack element.In this discussion, an integrated reforming stage and/or integratedendothermic reaction stage that represents an adjacent stack element toa fuel cell element is defined as being about one stack element heightor less away from the adjacent fuel cell element.

A thermal ratio of about 1.3 or less, or about 1.15 or less, or about1.0 or less, or about 0.95 or less, or about 0.90 or less, or about 0.85or less, or about 0.80 or less, or about 0.75 of less, can be lower thanthe thermal ratio typically sought in use of MCFC fuel cells. In aspectsof the invention, the thermal ratio can be reduced to increase and/oroptimize syngas generation, hydrogen generation, generation of anotherproduct via an endothermic reaction, or a combination thereof.

In various aspects of the invention, the operation of the fuel cells canbe characterized based on a thermal ratio. Where fuel cells are operatedto have a desired thermal ratio, a molten carbonate fuel cell can beoperated to have a thermal ratio of about 1.5 or less, for example about1.3 or less, or about 1.15 or less, or about 1.0 or less, or about 0.95or less, or about 0.90 or less, or about 0.85 or less, or about 0.80 orless, or about 0.75 or less. Additionally or alternately, the thermalratio can be at least about 0.25, or at least about 0.35, or at leastabout 0.45, or at least about 0.50. Further additionally or alternately,in some aspects the fuel cell can be operated to have a temperature risebetween anode input and anode output of about 40° C. or less, such asabout 20° C. or less, or about 10° C. or less. Still furtheradditionally or alternately, the fuel cell can be operated to have ananode outlet temperature that is from about 10° C. lower to about 10° C.higher than the temperature of the anode inlet. Yet further additionallyor alternately, the fuel cell can be operated to have an anode inlettemperature greater than the anode outlet temperature, such as at leastabout 5° C. greater, or at least about 10° C. greater, or at least about20° C. greater, or at least about 25° C. greater. Still furtheradditionally or alternately, the fuel cell can be operated to have ananode inlet temperature greater than the anode outlet temperature byabout 100° C. or less, or about 80° C. or less, or about 60° C. or less,or about 50° C. or less, or about 40° C. or less, or about 30° C. orless, or about 20° C. or less.

Operating a fuel cell with a thermal ratio of less than 1 can cause atemperature drop across the fuel cell. In some aspects, the amount ofreforming and/or other endothermic reaction may be limited so that atemperature drop from the anode inlet to the anode outlet can be about100° C. or less, such as about 80° C. or less, or about 60° C. or less,or about 50° C. or less, or about 40° C. or less, or about 30° C. orless, or about 20° C. or less. Limiting the temperature drop from theanode inlet to the anode outlet can be beneficial, for example, formaintaining a sufficient temperature to allow complete or substantiallycomplete conversion of fuels (by reforming) in the anode. In otheraspects, additional heat can be supplied to the fuel cell (such as byheat exchange or combustion of additional fuel) so that the anode inlettemperature is greater than the anode outlet temperature by less thanabout 100° C. or less, such as about 80° C. or less, or about 60° C. orless, or about 50° C. or less, or about 40° C. or less, or about 30° C.or less, or about 20° C. or less, due to a balancing of the heatconsumed by the endothermic reaction and the additional external heatsupplied to the fuel cell.

The amount of reforming can additionally or alternately be dependent onthe availability of a reformable fuel. For example, if the fuel onlycomprised H₂, no reformation would occur because H₂ is already reformedand is not further reformable. The amount of “syngas produced” by a fuelcell can be defined as a difference in the lower heating value (LHV)value of syngas in the anode input versus an LVH value of syngas in theanode output. Syngas (sg) produced LHV (sg net)=(LHV (sg out)−LHV (sgin)), where LHV (sg in) and LHV (sg out) refer to the LHV of the syngasin the anode inlet and syngas in the anode outlet streams or flows,respectively. A fuel cell provided with a fuel containing substantialamounts of H₂ can be limited in the amount of potential syngasproduction, since the fuel contains substantial amounts of alreadyreformed H₂, as opposed to containing additional reformable fuel. Thelower heating value is defined as the enthalpy of combustion of a fuelcomponent to vapor phase, fully oxidized products (i.e., vapor phase CO₂and H₂O product). For example, any CO₂ present in an anode input streamdoes not contribute to the fuel content of the anode input, since CO₂ isalready fully oxidized. For this definition, the amount of oxidationoccurring in the anode due to the anode fuel cell reaction is defined asoxidation of H₂ in the anode as part of the electrochemical reaction inthe anode.

An example of a method for operating a fuel cell with a reduced thermalratio can be a method where excess reforming of fuel is performed inorder to balance the generation and consumption of heat in the fuel celland/or consume more heat than is generated. Reforming a reformable fuelto form H₂ and/or CO can be an endothermic process, while the anodeelectrochemical oxidation reaction and the cathode combustionreaction(s) can be exothermic. During conventional fuel cell operation,the amount of reforming needed to supply the feed components for fuelcell operation can typically consume less heat than the amount of heatgenerated by the anode oxidation reaction. For example, conventionaloperation at a fuel utilization of about 70% or about 75% produces athermal ratio substantially greater than 1, such as a thermal ratio ofat least about 1.4 or greater, or 1.5 or greater. As a result, theoutput streams for the fuel cell can be hotter than the input streams.Instead of this type of conventional operation, the amount of fuelreformed in the reforming stages associated with the anode can beincreased. For example, additional fuel can be reformed so that the heatgenerated by the exothermic fuel cell reactions can either be (roughly)balanced by the heat consumed in reforming and/or consume more heat thanis generated. This can result in a substantial excess of hydrogenrelative to the amount oxidized in the anode for electrical powergeneration and result in a thermal ratio of about 1.0 or less, such asabout 0.95 or less, or about 0.90 or less, or about 0.85 or less, orabout 0.80 or less, or about 0.75 or less.

Either hydrogen or syngas can be withdrawn from the anode exhaust as achemical energy output. Hydrogen can be used as a clean fuel withoutgenerating greenhouse gases when it is burned or combusted. Instead, forhydrogen generated by reforming of hydrocarbons (or hydrocarbonaceouscompounds), the CO₂ will have already been “captured” in the anode loop.Additionally, hydrogen can be a valuable input for a variety of refineryprocesses and/or other synthesis processes. Syngas can also be avaluable input for a variety of processes. In addition to having fuelvalue, syngas can be used as a feedstock for producing other highervalue products, such as by using syngas as an input for Fischer-Tropschsynthesis and/or methanol synthesis processes.

In some aspects, the reformable hydrogen content of reformable fuel inthe input stream delivered to the anode and/or to a reforming stageassociated with the anode can be at least about 50% greater than the netamount of hydrogen reacted at the anode, such as at least about 75%greater or at least about 100% greater. Additionally or alternately, thereformable hydrogen content of fuel in the input stream delivered to theanode and/or to a reforming stage associated with the anode can be atleast about 50% greater than the net amount of hydrogen reacted at theanode, such as at least about 75% greater or at least about 100%greater. In various aspects, a ratio of the reformable hydrogen contentof the reformable fuel in the fuel stream relative to an amount ofhydrogen reacted in the anode can be at least about 1.5:1, or at leastabout 2.0:1, or at least about 2.5:1, or at least about 3.0:1.Additionally or alternately, the ratio of reformable hydrogen content ofthe reformable fuel in the fuel stream relative to the amount ofhydrogen reacted in the anode can be about 20:1 or less, such as about15:1 or less or about 10:1 or less. In one aspect, it is contemplatedthat less than 100% of the reformable hydrogen content in the anodeinlet stream can be converted to hydrogen. For example, at least about80% of the reformable hydrogen content in an anode inlet stream can beconverted to hydrogen in the anode and/or in an associated reformingstage(s), such as at least about 85%, or at least about 90%.Additionally or alternately, the amount of reformable fuel delivered tothe anode can be characterized based on the Lower Heating Value (LHV) ofthe reformable fuel relative to the LHV of the hydrogen oxidized in theanode. This can be referred to as a reformable fuel surplus ratio. Invarious aspects, the reformable fuel surplus ratio can be at least about2.0, such as at least about 2.5, or at least about 3.0, or at leastabout 4.0. Additionally or alternately, the reformable fuel surplusratio can be about 25.0 or less, such as about 20.0 or less, or about15.0 or less, or about 10.0 or less.

EXAMPLES

In various aspects, using an elevated target electrolyte fill level fora fuel cell can provide an unexpected increase in operating voltage andan operating lifetime benefit relative to using a standard target filllevel.

The unexpected increase in operating voltage can be illustrated incomparison with the voltage behavior of a molten carbonate fuel cellunder standard conditions. Table 1 shows voltage values during operationat beginning of life for fuel cells operated under various conditions.The fuel cells were 250 cm² in size. The target electrolyte fill levelfor the fuel cells corresponded to either 56 vol % of the cathode porevolume or 80 vol % of the cathode pore volume. It is noted that a targetcathode electrolyte fill level of 80 vol % corresponds to a combinedtarget electrolyte fill level of roughly 87 vol %. The operatingconditions corresponded to either conventional conditions (17 vol % CO₂in cathode input flow, 75% CO₂ utilization) or carbon capture conditions(4 vol % CO₂ in cathode input flow, 90% CO₂ utilization).

TABLE 1 Voltage versus Target Electrolyte Fill Level Voltage VoltageTargeted Cathode Fill Level (mV at 17% CO₂) (mV at 4% CO₂) 56%(standard) 792 (5x average) 763 80% 783 (3x average) 761

As shown in Table 1, at conventional conditions, increasing the targetcathode electrolyte fill level to 80 vol % of the cathode pore volumeresults in a decrease in operating voltage of more than 10 mV atbeginning of operation. Under carbon capture conditions, the differencein voltage between conventional target electrolyte fill and elevatedtarget electrolyte fill is smaller, but the conventional targetelectrolyte fill still results in a higher operating voltage atbeginning of life for a fuel cell under carbon capture conditions. Table1 shows that at standard operating conditions, there is a clearadvantage to operating with the standard target electrolyte fill levelof 50 vol % to 60 vol % of the cathode pore volume. This illustrates whyconventional understanding of molten carbonate fuel cells has settled onuse of a standard target fill level. Additionally, even at carboncapture conditions, if only the beginning of life operating voltage isconsidered, it would appear that operating with a standard target filllevel provides an advantage.

In contrast to Table 1, FIG. 4 shows the voltage behavior of moltencarbonate fuel cells operated under carbon capture conditions over aperiod of time. To generate the data shown in FIG. 4, molten carbonatefuel cells were operated to generate a current density of either 120mA/cm² or 150 mA/cm² with a cathode input flow containing between 4.0vol % and 5.0 vol % CO₂ and a CO₂ utilization of roughly 90%. The fuelcells either had a standard target electrolyte fill level (50 vol % to56 vol % of the cathode pore volume; roughly 63 vol % to 70 vol %combined target electrolyte fill level) or an elevated targetelectrolyte fill level (80 vol % or more of the cathode pore volume;roughly 87 vol % combined target electrolyte fill level).

As shown in FIG. 4, after a brief initial period, the operating voltagefor the fuel cells with the elevated target electrolyte fill level washigher than the operating voltage for the fuel cells with the standardtarget electrolyte fill level. (The first few hours of data points forthe standard target fill level at 120 mA/cm² are not shown in FIG. 4,but it is believed that the beginning of life voltage was briefly higherthan the corresponding elevated target fill level.) This shows that overtime, operating with a higher target fill level of electrolyte providedan unexpected operating voltage increase. This unexpected operatingvoltage increase was more pronounced at the higher current density of150 mA/cm².

It is believed that the improved operating voltage when using anelevated target electrolyte fill level at carbon capture conditions isdue in part to the increased loss of lithium in the fuel cell. Theincreased loss of lithium can be observed in several manners. Oneindication of the increased loss of lithium is the overall decrease inthe amount of electrolyte present in a molten carbonate fuel cell afterextended operation at carbon capture conditions.

Table 2 shows the relative fill level of electrolyte in various portionsof a molten carbonate fuel cell after operating the fuel cell for 2500hours at carbon capture conditions (˜4.0 vol % CO₂ in cathode inputstream, ˜90% CO₂ utilization). The target electrolyte fill level was astandard fill level of roughly 56 vol % of the cathode pore volumeand >90 vol % of the matrix volume. This corresponds to a combinedtarget electrolyte fill level of roughly 70 vol %. The results shown inTable 2 are relative to a baseline of a fuel cell operated atconventional conditions for 2500 hours. The fill level change is basedon the available pore volume in each portion of the fuel cell.

TABLE 2 Relative Electrolyte Fill Level after Operation at CarbonCapture Conditions Component Relative Fill Level Cathode −23 vol %Matrix  −5 vol % Anode −50 vol %

The increased loss of lithium is believed to be due in part to increasedincorporation of lithium into the cathode itself. This increasedincorporation of lithium into the cathode is illustrated in FIG. 5. FIG.5 shows inductively-coupled plasma mass spectrometry analysis (ICP-MS)of cathode structures after exposure in a test environment to lithiumunder various conditions. The cathode structures were composed of nickeloxide. The cathode structures were tested by exposing the cathodestructure, an electrolyte, and a cathode collector in an out-of-celltest apparatus to an environment that simulates an oxidizingenvironment. Several different oxidizing environments were used. A firstoxidizing environment corresponded to 0.5 vol % CO₂, 9 vol % O₂, and 10vol % H₂O, with the balance being N₂. A second oxidizing environmentcorresponded to 4.1 vol % CO₂, 9 vol % O₂, and 10 vol % H₂O, with thebalance being N₂. A third oxidizing environment corresponded to 18.5 vol% CO₂, 11.3 vol % O₂, 3.0 vol % H₂O, with the balance being N₂. It isnoted that the third oxidizing environment corresponds to conventionalmolten carbonate fuel cell conditions, while the first and secondoxidizing environments correspond to carbon capture conditions.

After exposure of the model fuel cell structures in the out-of-cell testapparatus to the various oxidizing environments, the composition of thecathodes was analyzed using ICP-MS to determine the lithium content. Asshown in FIG. 5, exposure of a fuel cell to conventional operatingconditions resulted in a cathode with a lithium content of less than 3.0wt %. Exposure of a fuel cell to an oxidizing environment with roughly4.0 vol % CO₂ resulted in a cathode with a lithium content of greaterthan 4.0 wt %. Exposure of a fuel to an oxidizing environment withroughly 0.5 vol % CO₂ resulted in a cathode with a lithium content ofbetween 9.0 wt % and 10 wt %. Based on the results in FIG. 5, use ofcarbon capture conditions resulted in a substantial increase in theamount of lithium incorporated into the cathode in the out-of-cell testapparatus. A similar increase in lithium incorporation into the cathodeis believed to occur during fuel cell operation.

The modification in fuel cell behavior can also be seen in the ohmicresistance exhibited by a fuel cell when operated under variousconditions. FIG. 6 shows results from measurement of ohmic resistanceover time for fuel cells operated under three types of conditions. Thefuel cells had a size of 6.24 in ×6.24 in (15.85 cm×15.85 cm). A firstcondition was operation under conventional operating conditions (˜18 vol% CO₂ in the cathode input stream, ˜75% CO₂ utilization) with a standardtarget electrolyte fill level (˜56 vol % of the cathode pore volume).This was considered as a baseline condition. All of the data shown inFIG. 6 was normalized to the ohmic resistance at this baselinecondition. Thus, the ohmic resistance for the baseline condition isshown as “1.0” in normalized units. A second condition corresponded tocarbon capture conditions (˜4.0 vol % CO₂ in the cathode input stream,˜90% CO₂ utilization) with a standard target electrolyte fill level. Athird condition corresponded to carbon capture conditions with anelevated target electrolyte fill level (˜80 vol % of the cathode porevolume). As shown in FIG. 6, at the carbon capture conditions, using theelevated initial electrolyte fill level substantially reduced the ohmicresistance of the fuel cell under carbon capture conditions.

ADDITIONAL EMBODIMENTS

Embodiment 1. A method for producing electricity in a molten carbonatefuel cell comprising a lithium-containing electrolyte, the methodcomprising: operating a molten carbonate fuel cell comprising an anode,a matrix, and a cathode with a cathode input stream comprising 10 vol %or less of CO₂ at an average current density of 120 mA/cm² or more and aCO₂ utilization of 60% or more, the molten carbonate fuel cell furthercomprising a combined target electrolyte fill level of 70 vol % or moreof a combined matrix pore volume and cathode pore volume.

Embodiment 2. The method of Embodiment 1, wherein operating the moltencarbonate fuel cell comprises operating at a measured CO₂ utilization of75% or more.

Embodiment 3. A method for producing electricity in a molten carbonatefuel cell comprising a lithium-containing electrolyte, the methodcomprising: operating a molten carbonate fuel cell comprising an anode,a matrix, and a cathode with a cathode input stream comprising CO₂ at anaverage current density of 120 mA/cm² or more and a CO₂ utilization of90% or more, the molten carbonate fuel cell further comprising acombined target electrolyte fill level of 70 vol % or more of a combinedmatrix pore volume and cathode pore volume.

Embodiment 4. The method of any of the above embodiments, i) wherein thecathode input stream comprises 5.0 vol % or less of CO₂, ii) wherein thecathode exhaust comprises 2.0 vol % or less of CO₂, iii) wherein themolten carbonate fuel cell is operated at a transference of 0.95 orless, or iv) a combination of two or more of i), ii), and iii).

Embodiment 5. The method of any of the above embodiments, wherein theelectrolyte comprises a non-eutectic mixture, or wherein the lithiumcarbonate content of the electrolyte is greater than a correspondingeutectic composition by 10 wt % or more.

Embodiment 6. The method of any of the above embodiments, wherein thecurrent density is 150 mA/cm² or more.

Embodiment 7. The method of any of the above embodiments, wherein themolten carbonate fuel cell is operated for a cumulative time of 50 hoursor more.

Embodiment 8. The method of any of the above embodiments, wherein atarget cathode electrolyte fill level comprises 85 vol % to 140 vol % ofthe cathode pore volume.

Embodiment 9. The method of any of the above embodiments, wherein thecombined target electrolyte fill level is 85 vol % to 128 vol %.

Embodiment 10. The method of any of the above embodiments, wherein atleast a portion of the combined target electrolyte fill level is storedin the cathode collector.

Embodiment 11. A molten carbonate fuel cell comprising: a cathodecollector, a cathode, a matrix, and an anode; and a lithium-containingelectrolyte, a combined target electrolyte fill level of thelithium-containing electrolyte corresponding to 85 vol % or more of acombined matrix pore volume and cathode pore volume.

Embodiment 12. The fuel cell of Embodiment 11, wherein the electrolytecomprises a lithium carbonate content that is greater than acorresponding lithium content in a corresponding eutectic mixture by 10wt % or more.

Embodiment 13. The fuel cell of Embodiment 11 or 12, wherein at least aportion of the combined target electrolyte fill level is stored in thecathode collector.

Embodiment 14. The fuel cell of any of Embodiments 11 to 13, wherein thecombined target electrolyte fill level is 90 vol % to 127 vol %.

Embodiment 15. The fuel cell of any of Embodiments 11 to 14, wherein thefuel cell comprises a target cathode electrolyte fill level of 85 vol %to 140 vol %.

Additional Embodiment A. The method of any of Embodiments 1 to 10 or thefuel cell of any of Embodiments 11 to 15, wherein the cathode porevolume is 1.5 to 2.0 times the matrix pore volume.

All numerical values within the detailed description and the claimsherein are modified by “about” or “approximately” the indicated value,and take into account experimental error and variations that would beexpected by a person having ordinary skill in the art.

Although the present invention has been described in terms of specificembodiments, it is not necessarily so limited. Suitablealterations/modifications for operation under specific conditions shouldbe apparent to those skilled in the art. It is therefore intended thatthe following claims be interpreted as covering all suchalterations/modifications that fall within the true spirit/scope of theinvention.

The invention claimed is:
 1. A method for producing electricity in amolten carbonate fuel cell comprising a lithium-containing electrolyte,the method comprising: operating a molten carbonate fuel cell comprisingan anode, a matrix, and a cathode with a cathode input stream comprising10 vol % or less of CO₂ at an average current density of 120 mA/cm² ormore and a CO₂ utilization of 60% or more, the molten carbonate fuelcell further comprising a combined target electrolyte fill level of 70vol % or more of a combined matrix pore volume and cathode pore volume.2. The method of claim 1, wherein operating the molten carbonate fuelcell comprises operating at a measured CO₂ utilization of 75% or more.3. The method of claim 1, wherein the cathode input stream comprises 5.0vol % or less of CO₂, or wherein a cathode exhaust comprises 2.0 vol %or less of CO₂, or wherein the molten carbonate fuel cell is operated ata transference of 0.95 or less, or a combination of two or more thereof.4. The method of claim 1, wherein the electrolyte comprises anon-eutectic mixture, or wherein a lithium carbonate content of theelectrolyte is greater than a corresponding eutectic composition by 10wt % or more.
 5. The method of claim 1, wherein the current density is150 mA/cm² or more.
 6. The method of claim 1, wherein the moltencarbonate fuel cell is operated for a cumulative time of 50 hours ormore.
 7. The method of claim 1, wherein a target cathode electrolytefill level comprises 85 vol % to 140 vol % of the cathode pore volume.8. The method of claim 1, wherein the combined target electrolyte filllevel is 85 vol % to 128 vol %.
 9. The method of claim 1, wherein atleast a portion of the electrolyte is stored in a cathode collector.